1. Field of the Invention
The present invention relates in general to dual-bandwidth cellular telephones, and more particularly to a dual-bandwidth cellular telephone switching apparatus for automatically switching power to two different cellular telephone systems to provide services on the cellular telephone systems.
2. Description of the Prior Art
Generally, two transceivers have been used to provide services on two different cellular telephone systems. Use of the two transceivers is both bulky and relatively expensive. In order to make up for such problem, it have been proposed to switch an intermediate frequency (referred to hereinafter as IF) section. Such an IF section switching method is a manual switching method for switching the IF section in response to user activation of a mechanical switch. However, the manual switching method is not suitable for use in cellular telephones and also requires user intervention and additional circuitry, increasing the cost and complexity thereof.
Another IF section switching method is an analog switching method which is used in most cellular telephones. One example of analog switching method is shown in U.S. Pat. No. 4,972,455. However, the analog switching method has a disadvantage in that it requires a relatively expensive frequency switch for passing a radio frequency (referred to hereinafter as RF) signal. A conventional analog switching apparatus using the above-mentioned analog switching method will hereinafter be described with reference to FIG. 1.
Referring to FIG. 1, there is shown a circuit diagram of a conventional dual-bandwidth cellular telephone switching apparatus. As shown in this drawing, the conventional dual-bandwidth cellular telephone switching apparatus comprises a signal receiver 100 and a switching circuit 200.
The signal receiver 100 includes an antenna 10 for receiving RF signals on a field corresponding to frequency bands of two different cellular telephone systems, a duplexer 11 for passing a desired frequency band component of output signals from the antenna 10, a low-noise amplifier 12 for amplifying a feeble signal from the duplexer 11, a band pass filter 13 for passing a desired frequency band component of an output signal from the low-noise amplifier 12, a synthesizer 14 for detecting a channel being presently serviced, and a mixer 15 for mixing an output signal from the band pass filter 13 with an output signal from the synthesizer 14 to produce an IF signal.
The switching circuit 200 includes a buffer 21 for buffering an output signal from the mixer 15 in the signal receiver 100, an analog switch 22 for selectively transferring an output signal from the buffer 21 to terminals A and B corresponding respectively to the two different cellular telephone systems in response to an A/B switching control signal s from a central controller 20, a first IF filter 23 for filtering an output signal from the terminal A if the terminal A is selected by the analog switch 22, a second IF filter 24 for filtering an output signal from the terminal B if the terminal B is selected by the analog switch 22, a first detector 25 for detecting an output signal from the first IF filter 23 to produce an audio signal and data, and a second detector 26 for detecting an output signal from the second IF filter 24 to produce an audio signal and data.
The operation of the conventional dual-bandwidth cellular telephone switching apparatus with the above-mentioned construction will hereinafter be described.
The RF signals are received by the antenna 10 and applied to the duplexer 11 which passes a desired frequency component thereof. The output signal from the duplexer 11 is amplified by the low-noise amplifier 12 and filtered by the band pass filter 13. The synthesizer 14 produces a local oscillating frequency using a phase locked loop (referred to hereinafter as PLL) and supplies the produced local oscillating frequency to the mixer 15 which is also applied with the output signal from the band pass filter 13. The mixer 15 mixes the output signal from the band pass filter 13 with the output signal from the synthesizer 14 to produce the IF signal. The mixer 15 then outputs the IF signal to the buffer 21. At this time, the central controller applies the A/B switching control signal s to the analog switch 22 to perform the switching operation.
First, the analog switch 22 operates responsive to the A/B switching control signal s from the central controller 20 to select the terminal A. As a result, the output signal from the buffer 21 is transferred to the terminal A by the analog switch 22. It is then checked whether the input signal at the terminal A is normal. If it is checked that the input signal at the terminal A is normal, the signal reception is continuously performed. If it is checked that the input signal at the terminal A is not normal, the analog switch 22 operates responsive to the A/B switching control signal s from the central controller 20 to select the terminal B. As a result, the output signal from the buffer 21 is transferred to the terminal B by the analog switch 22. Whenever the signal reception is performed, the above switching operation is repeated to selectively receive a digital or analog signal.
However, the above-mentioned conventional dual-bandwidth cellular telephone switching apparatus has a disadvantage in that the analog switch is relatively expensive, increasing the cost. Further, the analog switch cannot accurately perform the switching operation because of a loss in the signal transfer.